Magnetic circuit opening and closing motor starting switch in response to heat varying macnetic permeance

ABSTRACT

A synchronous motor and a starting circuit therefore in which a switch in series with the motor winding determines the starting period during which the starting winding is electrically energized. The switch has a leaf-spring movable contact tending toward opening the starting circuit kept in a circuit-closing position by a magnetic system which has a ferromagnetic member therein rendered nonmagnetic by heat applied by a PTC-resistor in series with the motor starting winding so that the ferromagnetic member releases a magnet on the movable contact closing the switch and the motor starting period is thus determined.

United States Patent [72] Inventor Erik Kurt Poulsen Egelykke, Vollerup, Denmark [21] Appl. No. 762,757 [22] Filed Sept. 26, 1968 [45] Patented Feb. 23, 1971 [73] Assignee Danfoss A/S Nordborg, Denmark [32] Priority Oct. 7, 1967 1 Germany [31] Pl588139.7

[54] MAGNETIC CIRCUIT OPENING AND CLOSING MOTOR STARTING SWITCH IN RESPONSE TO HEAT VARYING MAGNETIC PERMEANCE 4 Claims, 1 Drawing Fig.

[52] U.S.Cl 318/221; 335/146 '51] mcr ..I-I02k 17/08 [50] FieldofSearch 318/221, 221.6,221.8,221.9;335/146 [56] References Cited v UNITED STATES PATENTS 2,261,250 11/1941 l-laddad 172/278 2,668,884 2/1954 Jacobs 335/146 2,751,483 6/1956 Keen et al.... 335/146 3,284,736 ll/l966 Vance 335/146 3,430,166 2/1969 Gambill... 335/146 2,117,123 5/1938 Werner 318/2216 2,588,257 3/1952 Lewus 3l8/22l.8

Primary Examiner-Cris L. Rader Assistant Examiner-K. L. Crosson Attorney-Wayne B. Easton MAGNETIC CIRCUIT OPENING AND CLOSING MOTOR STARTING SWITCH IN RESPONSE TO BEAT VARYING MAGNETIC PERMEANCE This invention relates generally, to single-phase asynchronous motors and more particularly to a starting switch for a single-phase motor in the starting branch circuit thereof.

A starting switch for single-phase asynchronous motors is known which is actuated electromagnetically by the main winding of the motor in dependence upon the current and opens when this current falls below a certain value. Also there is known a starting switch having a temperature-dependent component, eg a bimetallic element which is heated during the starting operation, so that the switch opens some time after the commencement of the starting operation. A starting switch is also known wherein only a FTC-resistor, i.e. a resistor, the resistance of which increases with temperature, is connected in the starting branch circuit. This is heated by the starting current and consequently assumes a resistance of a magnitude such that the current through the starting winding is negligible small. However, whereas the actuation of the two first-named switches calls for considerable expense (the provision of a magnet winding or heating winding), in the case of the last-mentioned switching deviceit is notalways possible to keep the cold resistance of the FTC-resistor so low that a sufficiently high starting current can flow in the starting circuit.

According to the invention a starting switch is kept closed by means of a magnetic system which consists of a permanent magnet and a yoke which bridges its poles and is made of a ferromagnetic material that loses its magnetic conductivity and is rendered nonmagnetic upon being heated. The yoke is heated by a'PTC-resistor, which is connected in parallel with the switch and a resistor arranged in series therewith. When the ferromagnetic materialis rendered nonmagnetic the starting switch opens.

In this arrangement, the starting switch comprises a movable contact kept closed by the fully effective magnet. Consequently, the starting current is limited only by the series resistor in series with the starting switch. The value or rating of the series resistor can be selected asrequired, and, of course,

the starting current is affected by the resistance of the starting winding itself. However, when the motor is started current also flows through thebridging FTC-resistor. The PTC-resistor heats the yoke of the magnetic system. When the Curie point of the ferromagnetic yoke is reached, it is rendered nonmagnetic and the switch opens. r e v The length of the starting period of the motor can be influenced by various factors, for example by the choice of different ferrites having different Curie points, by the choice of the FTC-resistor material, by the choice ofthe thermal capacity of the heating resistor and the yoke, by the nature of the heat transfer, and so forth. it is particularly advantageous if the yoke and the FTC-resistor are interconnected in a thermally conductive or best heat-transfer manner, since then the smallest possible amount of heat suffices to bring the yoke to the Curie point. j A very simple construction of the starting switch is obtained if the moving contact of the switch is a'pretensioned leaf spring. The spring force should be such'that the magnet, in the cold condition, or deenergized condition of the motor is able to close the'switch and to keep it closed. In a preferred embodiment of theiinvention the permanent magnet is disposed on the movable contact of the switch and the yoke/PTC-resistor combination is stationary. I

Other features and advantages of the starting circuit in accordance with the present invention will be better understood as described in the following specification and appended claims in conjunction with the sole drawingconstituting a schematic of amotor provided with a starting circuit according to the invention.

According to the drawing a main winding 4 is connected by a main switch 3 to two supply terminals 1 and 2, and a starting starting switch 7 are connected in series with the motor starting winding. A contact 8 of the starting switch is attached to an upwardly pretensioned spring 9 jointly forming therewith the movable contact of the switch. Apermanent magnet 10 is attached to the leaf spring 9. I

This magnet together with a ferromagnetic member comprising a ferrite plate 11, form a magnetic system. The ferrite plate 11 is mounted in a thermally conductive relationship with a FTC-resistor 12 which is arranged electrically in parallel with the series connection of the series resistor 6 and the starting switch 7. The ferrite plate 11 possess the property of losing its magnetic permeance when it is heated beyond the Curie point.

When the main switch 3 is closed the full short circuit current flows through the main winding 4. A starting current flows through the starting winding 5, which is largely determined by the resistance of the starting winding and the series resistor 6. At the same time, current flows through the PTC- resistor 12, so that it is heated and with it the ferrite plate 11. After some time, which depends upon the particular characteristics of the ferromagnetic material and the particular construction,. the temperature of the plate 11 rises above the Curie point and the switch 7 opens, since-the leaf spring 9 is no longer held in an attracted condition by. the magnet system. Thus, the starting operation or period terminates and the starting current drops to a very low value, which is mainly determined by the thermal resistance of the FTC-resistor 12. Only a small starting current flows, therefore, but this suffices to keep the magnetic system 10, 11 in a nonmagnetic condition.

While a preferred embodiment of the invention has been shown and described it will be understood that'many modifitrically disconnectsaid starting winding upon completion of said starting period, a movable contact on said switch, a magnetic system comprising a permanent magnet releasably closing said switch mounted on said movable contact, said movable contact being magnetically actuated by said magnet to a switch-closing position and automatically operable to a switch-opening position when magnetic system attraction of said magnet is reduced sufficiently to allow said contact to open said switch, a stationary magnetizable member cooperating with said magnet to define a magnetic circuit for causing said magnet to close said switch, and means to reduce the permeance of said magnetizable member thereby to reduce magnetic attraction by said magnetic system to cause said magnet to effect release of said movable contact and determine said starting period.

2. A starting circuit for a single-phase asynchronous motor according to claim 1, in which said magnetizable member comprises a ferrite.

according to claim 3, in which said means electrically. .en'e'r gized comprise a positive temperature coefficient resistor in series with said main winding and in parallel with the starting winding heating said member above the Curie point thereof.- 

1. A starting circuit for a single-phase asynchronous motor having a main winding and a starting winding, said starting circuit comprising means for electrically energizing said starting winding and said main winding, a switch in series with said starting winding for keeping said starting circuit closed and said starting winding in circuit during a predetermined starting period and subsequently opening said starting circuit to electrically disconnect said starting winding upon completion of said starting period, a movable contact on said switch, a magnetic system comprising a permanent magnet releasably closing said switCh mounted on said movable contact, said movable contact being magnetically actuated by said magnet to a switch-closing position and automatically operable to a switch-opening position when magnetic system attraction of said magnet is reduced sufficiently to allow said contact to open said switch, a stationary magnetizable member cooperating with said magnet to define a magnetic circuit for causing said magnet to close said switch, and means to reduce the permeance of said magnetizable member thereby to reduce magnetic attraction by said magnetic system to cause said magnet to effect release of said movable contact and determine said starting period.
 2. A starting circuit for a single-phase asynchronous motor according to claim 1, in which said magnetizable member comprises a ferrite.
 3. A starting circuit for a single-phase asynchronous motor according to claim 1, in which said means to reduce the permeance of said member comprises means electrically energized heating said member.
 4. A starting circuit for a single-phase asynchronous motor according to claim 3, in which said means electrically energized comprise a positive temperature coefficient resistor in series with said main winding and in parallel with the starting winding heating said member above the Curie point thereof. 